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- Small intestinal surface area is maximized by the valvulae conniventes, villi, and microvilli.
- Proteins and carbohydrates are polymers which must be hydrolysed (adding water) by salivary, gastric, pancreatic, and brush border enzymes to yield monomers (or, occasionally, dimers) which can then be transported by special mechanisms through the lipoidal cell membranes.
- Lipophilic (non-polar) molecules need to be transported within mixed micelles to cross the unstirred juxtamucosal water layer before they can be passively abosorbed.
- Small intestinal motility consists of three components: segmental contraction which retard and mix luminal contents; peristalitc contractions which propel luminal contents distally for short distances; interdigestive housekeeping waves which clear the small intestine for next meal.
- The upper small intestine serves to render meals isotonic (290 mosmols/kg) with plasma by having a relatively permeable epithelium, and by coupling the absorption of Na+ with nutrient monomer. Water follows passively the direction of net Na+ transport.
- Transport of water and electrolytes is affected by intestinal motility and blood flow, and by intrinsic autoregulation of absorption and secretion. Bacterial and viral toxins can block Na+ absorption, and stimulate Cl- secretion. Often, the Na+/glucose cotransport is unaltered so that hydration can be maintained by providing glucose-electrolyte solutions to afflicted patients.
- Malabsorption of dietary triglyceride can be caused by too rapid gastric emptying, by defects in luminal digestion and solubilization, and by abnormalities in mucosal processing.
- Most nutrients (Ca++, Fe++, Zn++, water-soluble, and fat-soluble vitamins) are absorbed in the duodeno-jejunum. Vitamin B12, and most of the bile salts are absorbed in the ileum which can assume the absorptive functions of the proximal small intestine if the latter is restricted.
- Coefficient of intestinal absorption = (Oral intake – fecal output) ÷ oral intake. Insorption is movement of a substance from lumen to blood (or from extracellular to intracellular). Exsorption is movement from blood to lumen (or from intracellular to extracellualtr).
- Synopses of assimilation of major nutrients.
| Carbs: |
| Stage |
Main Structure |
Key
Factor(s) |
Example of a Disorder |
Physiology, Pathophysiology |
| Digestion & Solubilization |
Stomach |
Salivary amylase |
Sicca syndrome |
Decreased secretion |
| |
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Emptying |
Vagotomy |
Loss of metered delivery |
| |
Pancreas |
Pancreatic Amylase |
Pancreatic loss ZE syndrome |
Low amylase output Acidic pH inhibits |
| |
Enterocyte |
Brush border Enzymes |
Congenital |
Glucose-galactose malabsorption |
| |
|
|
Celiac sprue |
Decreased surface Brush border enzyme deficiency |
| |
Colon |
Bacteria |
Antibiotic induced diarrhea |
Water-soluble carbs can't be metabolized to absorbable SCFA |
| Insoprtion |
Duodeno-
jeijunum |
Enterocytes |
Celiac sprue |
Decreased surface
Injured brush border |
| |
Colon |
Bacteria; colonocytes |
Rapid transit |
Insufficient time for bacterial metabolism of carbs and for absorption of SCFA |
| Transport |
Portal venous system |
Hepatic storage |
Cirrhosis |
Absorbed carbs and SCFA by-pass liver |
Assimilation of Carbohydrates (ingested as polysaccharides such as starch;
disaccharides, sucrose, lactose; monosaccharides, glucose, fructose)
ZE, Zollinger-Ellison; Carbs, carbohydrates
| Protein: |
| Stage |
Main Structure |
Key
Factor(s) |
Example of a Disorder |
Physiology, Pathophysiology |
| Digestion & Solubilization |
Stomach |
Pepsins |
Gastric atrophy |
Decreased secretion |
| |
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Emptying |
Vagotomy |
Loss of metered delivery |
| |
Pancreas |
Pancreatic proteases |
Pancreatic loss ZE syndrome |
Low enzyme output
Acidic pH inhibits |
| |
Enterocyte |
Brush border Enzymes |
Congenital |
Malabsorption of specific amino acids |
| |
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|
Celiac sprue |
Decreased surface Brush border enzyme deficiency |
| |
Colon |
Bacteria |
Antibiotic induced diarrhea |
Amino acids can't be metabolized to absorbable SCFA |
| Insoprtion |
Duodeno-
jejunum |
Enterocytes |
Celiac sprue |
Decreased surface
Injured brush border |
| |
Colon |
Bacteria; colonocytes |
Rapid transit |
Insufficient time for bacterial metabolism of amino acids and for absorption of SCFA and ammonia |
| Transport |
Portal venous system |
Hepatic storage |
Cirrhosis |
Absorbed amino acids, SCFA, and NH3 by-pass liver |
Assimilation of Protein (ingested as polypeptides; amino acids)
ZE, Zollinger-Ellison
| Fat: |
| Stage |
Main Organ |
Key
Factor(s) |
Example of a Disorder |
Physiology, Pathophysiology |
| Digestion |
Stomach |
Gastric lipase |
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Important in the newborn; in low pancreas output |
| |
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Emptying |
Surgery |
Loss of metered delivery |
| |
Pancreas |
Lipase,
co-lipase |
Pancreatic loss
ZE syndrome |
Low amylase output
Acidic pH inhibits |
| Solubilization |
Liver,
biliary tract |
Conjugated bile salts |
Chronic hepatitis Obstructed bile SBBO |
Impaired synthesis Impaired delivery Deconjugation |
| Insoprtion |
Duodeno-
jeijunum |
Enterocytes |
Celiac sprue |
Decreased surface
Injured brush border |
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Golgi apparatus |
abetaliproteinemia |
Decreased synthesis of chylomicrons |
| Transport |
Lymphatics |
Lacteals |
Whipple disease |
Lacteal obstruction |
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Lymphatic ducts |
Retroperitoneal fibrosis |
Lymphatic duct obstruction |
Assimilation of Dietary Fat (ingested as triglycerides, phospholipids, etc)
ZE, Zollinger-Ellison; SBBO, small bowel bacterial growth
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